In women with a healthy vaginal microbiome, lactobacillus communities acidify the vagina with lactic acid, a potent and broad-spectrum microbicide that reinforces the mucosal barrier to sexually transmitted infections (STIs). However, for at least one-third of women worldwide (and >50 percent of black women), protective lactobacilli are displaced by a dense mixture of Gram-variable and Gram-negative bacteria, often leading to a condition known as bacterial vaginosis (BV). BV markedly increases susceptibility to virtually all STIs, including HIV. Although BV can be treated with antibiotics, relapse occurs frequently, with ~2 BV episodes per month. How vaginal bacterial communities shift rapidly between protective lactobacilli and unprotective BV is not understood, and no method currently exists for preventing BV. This project seeks to identify factors that cause the rapid shifts between vaginal bacterial communities. The results may guide the development of much needed methods to prevent BV and thereby substantially reduce STI transmission rates. The overall hypothesis is that these shifts result from antagonistic actions between lactobacilli and BV-associated bacteria that are mediated by secreted inhibitory factors; preliminary evidence suggests lactic acid secreted by lactobacilli, and amines secreted by BV microbes, may be two such factors. To test this hypothesis, qPCR analysis will be used to quantify the specificity and potency of inhibitory factors present in ex vivo healthy, lactobacillus-dominated vaginal secretions against BV-associated bacteria, and conversely factors in ex vivo BV secretions that inhibit lactobacilli (Aim 1). In addition, the dynamics of BV onset (Aim 2) will be investigated by monitoring the depletion of lactobacilli, and the decrease in inhibition of BV-associated bacteria in vaginal secretions during menses or following vaginal douching (two events associated with BV onset). Similarly, BV remission (Aim 3) will be studied by monitoring antagonistic inhibitory factors in vaginal secretions as lactobacilli recover following a BV episode. The use of fresh, minimally-perturbed ex vivo vaginal secretions to investigate the actions of inhibitory factors represents a significant innovation; most prior research has been performed in vitro in growth media, which fails to capture the unique mixture of cells (including symbiotic microbial communities), inhibitory factors and mucus fluid present in vivo.